1. Field of the Invention
The present invention relates to a an image forming apparatus such as copier, a printer, and a fax machine, and more particularly to a developing device and a process cartridge used in an image forming apparatus.
2. Description of the Related Art
In known conventional one-component developer devices, for instance as disclosed in Japanese Patent No. 3320954, non-magnetic or magnetic toner (developer) stored in a container is supplied to a developing roller (developer carrier) by means of a toner supply member (roller) made of, for instance, foamed polyurethane or the like, and an elastic member such as a metal thin plate or the like is pressed and brought into contact with the developing roller (developer carrier) to homogenize thereby the toner on the developing roller (developer carrier) and develop an electrostatic latent image formed on a photosensitive member (image carrier).
In such developing devices, toner is scraped by a doctor blade. Herein, repeated passes over the doctor blade promotes eventual deterioration of the toner. When the toner, which is spread as a uniform thin film on the developing roller, is not consumed in the image, it returns again into the developing device. A problem in such devices is that friction at the scraping portion (pressing portion) of the toner supply member is substantial, which accelerates degradation of the toner (impairing, for instance, toner chargeability).
In recent years color image forming apparatuses have become widespread in the wake of the growing share of color documents in the office. These color image forming apparatuses include apparatuses that comprise four photosensitive drums in tandem. Four sets of image forming mechanisms, having each respective image forming functions, are built into the image forming apparatus. Developer images (toner images) of a respective color are formed on the four photosensitive drums, as image carriers, using a powdery one-component yellow, magenta, cyan or black developer (toner). The developer images are then sequentially transferred onto one sheet of a transfer material, to obtain a color image.
In conventional developing devices, however, the toner charging portion, the toner layer forming portion and the developer storage portion are disposed side by side (for instance, as disclosed in Japanese Patent No. 3320954). Such developing devices are thus difficult to make smaller, which is a serious drawback, in particular, for reducing the size of a color image forming apparatus comprising a plurality of such developing devices.
Therefore, size reduction has been realized by using an elongated developing device, as disclosed in, for instance, Japanese Patent Application Laid-open No. 2003-5487 (FIG. 7). In such an elongated developing device, toner is supplied as needed, sinking downwards on account of it own weight, to a developing roller, positioned below the developing device, that consumes the toner. At the lower portion of the device, toner that has become degraded by passing through the above-described friction portions is mixed with non-degraded toner that occasionally sinks from a storage section above. The various toner flows are mixed herein through the action of rotating members such as the developing roller, the supply roller and so forth, as a result of which there occur no sudden toner changes.
By contrast, toner accumulates without mixing in portions where flow is non-existent on account of remoteness from rotating bodies, among other factors. Virtually non-degraded toner accumulates, for instance, above the doctor blade, where the toner has nowhere to go and is substantially beyond the influence of rotating members. When the life of the device is drawing to its close and a certain amount of toner is consumed in the device, the own weight of toner above the doctor blade causes the accumulated toner to sink. Toner from that portion, which is virtually unmixed with degraded toner, may penetrate directly into the nip of the elastic member and become charged on the developing roller. The device deteriorates gradually as its life wears on, and toner charge on the developing roller changes gradually as well. In that situation, however, the intrusion of toner accumulated above the doctor gives rise to the problem of abrupt variations in toner charge on the developing roller. In particular, when the developer is a one-component developer, which unlike a two-component developer has no magnetic carrier, the behavior of toner, as the developer, inside the developing device becomes reflected on the images.